This paper is concerned with the development of a modelling framework to predict the effects of welding and post-weld heat treatment on thereto-mechanical performance of welded material, as a step towards a design tool for industry. A dislocation mechanics, through-process finite element model, incorporating thermal, micro structural and mechanical effects is presented, for predicting thereto-mechanical fatigue of welds. The model is applied to multi-pass gas tungsten arc welding of 9Cr martensitic steel. The predicted high-temperature low cycle fatigue performance of cross-weld samples is comparatively assessed for a range of different post-weld heat treatment durations. It is shown that longer post-weld heat-treatment (PWHT) durations increase the predicted number of cycles to failure and that Vickers hardness gradient across the heat-affected zone can be used as an indicator of fatigue life.